Three Fingered Jack: A dissected mafic composition volcano in the central Cascades

Javaria Aziz

Portland State University

The central Oregon High Cascades are characterized by intra-arc extension and voluminous mafic lava flows erupted from scoria cones, shield volcanoes, and mafic composite cones. Three Fingered Jack is an eroded, large mafic composite volcano that stands out among smaller monogenetic cinder cones and shield volcanoes. It has been deeply dissected from glaciation, exposing much of its eruptive stratigraphy. Complete sequences of stacked, thin lava flows, are exposed on the southern and eastern flanks. The core of the edifice is highly stratified with layers of pyroclastic flows, scoria deposits and lava flows. The edifice core is cut by numerous dikes that are fed from intrusive plugs. In this talk, we will explore the eruptive products of Three Fingered Jack and discuss the changes in lava composition from a regional perspective.

Please join us at:

Worthy Brewing (eastside location)

In the Hop Mahal Room

5:30 pm social hour

7:00 pm presentation

COGS talk are free and open to the public -- all are welcome! Please join us for the social hour before the presentation.

All presentations are also live-streamed through Zoom. There will be a registration link at the top of this page as we get closer to the date of this presentation.